This PR implements deprecation of thebaseUrl compiler option, which will be removed in TypeScript 7.0. ThebaseUrl option currently performs two functions that can be confusing to users:

1. Acts as a prefix for all entries inpaths
2. Acts as a fallback resolution point for bare module imports (often unintended)

Problem

When users configure:

{"compilerOptions": {"baseUrl":"./src","paths": {"@app/*": ["app/*"],"@lib/*": ["lib/*"]    }  }}

An import likeimport * as blah from "blah.js" might resolve to:

• ./src/app/blah.js
• ./src/lib/blah.js
• ./src/blah.js⚠️ (often undesirable)

The last resolution path is usually unintentional and can cause confusion.

Solution

Starting in TypeScript 6.0, usingbaseUrl will show a deprecation warning:

Option 'baseUrl' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.

Users should migrate to explicit path mappings:

Before:

{"compilerOptions": {"baseUrl":"./src","paths": {"@app/*": ["app/*"],"@lib/*": ["lib/*"]    }  }}

After:

{"compilerOptions": {"paths": {"@app/*": ["./src/app/*"],"@lib/*": ["./src/lib/*"]    }  }}

To preserve baseUrl fallback behavior (optional):

{"compilerOptions": {"paths": {"*": ["./src/*"],"@app/*": ["./src/app/*"],"@lib/*": ["./src/lib/*"]    }  }}

Implementation

• Added deprecation check insrc/compiler/program.ts using existing deprecation infrastructure
• SupportsignoreDeprecations: "6.0" to silence warnings during transition period
• Added comprehensive test coverage including migration examples
• Updated 70 existing tests to useignoreDeprecations to prevent test breakage
• All existing functionality preserved - this is a warning-only change

Fixes#62207.

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