Targets and Rules
.PHONY targets ignore pattern rules
Make is designed to assume that .PHONY targets are abstract commands (like clean, install, or test), not files derived from other files. When Make sees that clean_c needs bar.o, and bar.o is phony, it skips your %.o: %.c pattern rule entirely. It searches exclusively for an explicit rule named exactly bar.o:. Because you don't have one, it stops instantly:
Explicit/Pattern Targets that are empty have different behaviours!
1. Explicit Target (a.o: a.c) = "Add a Dependency"
If you write an explicit rule with no recipe:
This is how people add header file dependencies (e.g., a.o: a.h) without having to rewrite the whole gcc compilation commands. Make just merges this extra dependency with its built-in knowledge. And then it will look through its database of implicit rules (like the built-in C compiler rules) to find the recipe.
2. Pattern Target (%.o: %.c) = "Modify the Database"
If you write a pattern rule with no recipe:
In GNU Make, defining a pattern rule with no recipe is the official syntax for deleting/canceling a rule. By writing those lines, you are explicitly telling Make to forget any built-in knowledge it has about turning .c files into .o files. Even if you removed the .PHONY tag to fix the first problem, Make still wouldn't know how to connect bar.o to bar.c because you essentially erased the bridge connecting them.
Defining the same target multiple times
If you define the same target multiple times with different prerequisites, it accumulates (aggregates) them into a single, master list. e.g., using multiple .PHONY.
But for recipes, Make doesn't merge them. Instead, the last recipe parsed wins.
Implicit Rules
Implicit rule search happens whenever a target has NO RECIPE (commands). (No rules or Rules with no recipes).